Bimaterial thermosensitive element



Dec. 10, 1968 BARKER, JR 3,415,712

BIMATERIAL THERMOSENSITIVE ELEMENT Filed Oct. 51, 1963 2 Sheets-Sheet 1lm/emor 9012 en E. Burke/Mn,

Dec. 10, 1968 R. E. BARKER, JR

BIMATERIAL THERMOSENSITIVE ELEMENT 2 Sheets-Sheet 2 Filed Oct. 31) 1963Fig. 2.

IIIIII] lll'llll Thickness Rat/a (m/ I l I I II 6; KL, 5 /nven/or:Haber) E. Bar/var, Jr.

Raf/o [last/c Moduli (/1) His Affomey- United States Patent 3,415,712BIMATERIAL THERMOSENSITIVE ELEMENT Robert E. Barker, Jr., Schenectady,N.Y., assignor to General Electric Company, a corporation of New YorkFiled Oct. 31, 1963, Ser. No. 320,440 6 Claims. (Cl. 161183) Thisinvention relates to improvements in the construction of bimaterialtemperature-sensitive strips useful in thermomicroswitches,temperature-sensitive capacitors and temperature indicating devices ingeneral and in the construetion of the microminiaturized versions ofsuch devices in particular.

Both bimetallic and bimaterial temperature-sensitive constructions areknown in the prior art, but the sensitivity of these earlierconstructions is, in each instance, substantially less than the responsethat theoretically can be made available from the coupled materials ofdiffering linear thermal expansivities.

Investigation has shown that many variables are involved in thefunctional relation between the components of a bimaterial or bimetallicstrip in effecting thermal deflection and whereas the parameter that ismost often emphasized is the difference in the linear thermalexpansivities of the two materials chosen (Aa=u a a thorough analysishas shown that the ratio of elastic moduli (n=E /E the ratio of materialthicknesses (m=a /a and the total strip thickness (h=a +a must beconsidered limiting factors of substantial importance. It has furtherbeen established that AOL and It cannot be freely chosen independentlyof each other and to provide an optimum bimaterial construction both ofthese parameters should be considered in selecting the proper pair ofmaterials. To some extent, the effect of difference in the elasticmoduli of the materials may even be compensated for by proper adjustmentof the value of m.

In the past the design of bimetallic and bimaterial strips have beenlimited for the most part to consideration of the coefficients ofexpansion of the two materials whose interaction is being relied upon toeffect deflection in response to changing temperatures. Apparently inthe development of bimaterial strips not only has there been a lack ofappreciation of the sizeable effect of the aforementioned parameters, n,m and h, on the system, but also, in each instance an additionalcomponent is introduced to the erstwhile two-component system in theform of a bonding agent or an external bonding mechanism. When thetwo-component system is complicated by the introduction of some externalbonding mechanism, as for example, rivets, cement, glue, etc., severalobjectionable aspects are introduced; namely, the fabrication of thetemperature-sensitive strip is more complicated, the deflection perdegree temperature change for the completed strip is reduced, changeswhich occur in the properties of the adhesive layer with time maynecessitate periodic recalibration and the presence of either rivets oran extra lamina in the form of a layer of adhesive greatly complicatesthe design considerations, extending far beyond the complexity of atwo-component structure.

Thus, an object of the present invention is the provision of a truebimaterial strip construction for use as a temperature sensing elementwherein the construction consists solely of a two-layer system.

Another object of the present invention is to provide a method forconstructing a bimaterial strip properly integrating the two-componentsto enable the composite strip to function as an accurate sensor oftemperature without the introduction of an additional component or layerto the system to effect the integration.

A further object of this invention is the provision of an economicaltemperature-sensing construction more sensitive than those heretoforeavailable in the art.

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These and other objects may be obtained in the present invention whereina properly chosen pair of different materials, at least one of which isa thermoplastic, upon being subjected to the proper surface treatmentand then being heated in contact with each other, will join duringcooling thereof to form an integral bimaterial temperature-sensingelement for use in temperature indicating devices and intemperature-sensitive electrical components.

The exact nature of this invention will be readily apparent fromconsideration of the following specification related to the annexeddrawings in which:

FIGS. 1a, 1b and 1c are schematic illustrations of a bimaterial stripconstructed according to this invention having notations thereon forreference in the analysis of the effect of the various parametersdeterminative of the curvature of such a strip under the effect oftemperature changes;

FIG. 2 is an array of a series of graphs plotted of the values of f(n,m) as a function of m for various positive real values of 11;

FIG. 3 is an array of a series of graphs plotted of the values of f(n,m) as a function of n for various positive real values of m;

FIG. 4 is a schematic representation of such a bimaterial strip mountedto provide, after proper calibration, an accurate temperature indicatingdevice; and

FIG. 5 shows the application of a bimaterial strip constructed accordingto this invention as a thermosensitive electrical device.

In order to illustrate the effect of the ratio of the elastic moduli andthe ratio of component material thickness on the thermal deformation ofbimaterial strips, reference is made to FIG. 1 and the notationsthereon.

These notations represent the following properties and dimensions of thebimaterial strip 10 composed of laminae 11 and 12, which are differentmaterials:

(1) all symbols having the subscript 1 refer to the upper layer ofmaterial, lamina 11;

(2) all symbols having the subscript 2 refer to the lower layer ofmaterial, lamina 12;

(3) the symbols employed have the following connotation: r (radius), AL(segment of length, F (force), M (bending moment), or (linear thermalexpansivity), a (thickness), b (width), h (height), T (temperature, andI (moment of inertia).

At some reference temperature T the strip 10 would be straight(r=infinity). If (1 is greater than 11 the curvature displayed in FIG. 1corresponds to a positive temperature change .AT=T (ambient temperature)-T Considering element AL of bimaterial strip 10 and the stress actingthereon it may be seen that the uniform bending moments, M and M actingthereon are equal and opposite as are the forces F and F which act onthe lengths AL of the respective laminae 11, 12 of the element 10. Alongthe interface 13 it is assumed that both components 11 and 12 alwaysmaintain a common length and are not displaced relative to each other.Upon exposure to an increase in temperature AT, the length of the uppercomponent 11 expands by the quantity In addition, the upper component 11is also stretched an extra amount (F /E a b)AL along its central axisdue to the expansion of the lower component 12, which is attempting toexpand by the amount oL AT'AL Actually the upper component 11 restrainsthe lower component 12 whereby component 12 experiences a compression (F/E a b)AL. This is represented in FIG. 1(a) as a shortening by thedistance a AL/2r or expressed an- F F E1011? In the absence of externalforces, one can see from the symmetry of the problem that F =F =Fintorface al and M +M =ttal couple:F(a +a /2 (3) From the elementarybeam theory, the bending moment M is related to the fiexural modulus EIand the curvature l/ r by the relation M=El/r where E is the tensilemodulus and, for a beam of rectangular cross-section ab, the secondmoment of area is given by I=a b/l2 Combining Equations 3 and 4:

E 1 E I E E In Eq. 1 and in Eq. 6, r wl'zwr (provided r h) so thatFollowing the introduction of dimensionless variables n=E /E m=a /a andthe use of the relations a =mh/(m+l) and a =h/(m+1), Eq. 9 may beexpressed:

The term in brackets will be denoted by (n, m). Equation 10, andtherefore (n, m.) applies to virtually all situations involving onedimensional bending of bimaterial structures by thermal forces. As anapproximation, it applies to some two dimensional problems. The directdependence of curvature (k) on a ot and on AT are results which arenormally expected. Likewise, an inverse dependence on It is fairlyplausible. Due to the more complicated form of (n, m) it is necessary toconsider its functional properties in some detail in order to see howl/r depends on elasticity and thickness ratios.

From this rather complex analysis one sees that many variables areinvolved in the functional relation determinative of thermal deflectionin a bimaterial strip and that although the quantity that has beenpredominantly considered in the past in such design is .Aa:a -a (i.e.,the difference in linear thermal expansivities of the two mate- Irials), it may be appreciated from the above analysis that Furtheranalysis has established that the maximum value of (n, m) is 1.5 and aseries of curves may be plotted of the values of (n, m) as a function ofm: for various positive real values of 11 (FIG. 2) and, as well, aseries of curves may be plotted of the values of f(n, m) as a functionof n for various positive real values of m (FIG. 3). These curvesprovide useful tools for designing bimaterial strips having maximumunrestrained curvature.

For example, if h and m are predetermined factors so that the thicknessof the laminae, but not the specific materials are fixed, then atentative choice of materials available in the requisite thicknesses tosatisfy the demands for h and m may be made such as to provide a largevalue for Act. This will then determine n and if the location of theintersection of the value for m with the curve for the chosen value of nis close to a value of 1.5 (the maximum) for fln, m) which is the casewhen n=m", then a good selection of materials has been made. If not,another selection of materials must be made. Ordinarily the choice ofmaterials yielding a large value for Act will also provide a large valuefor n.

If, on the other hand, neither 11 nor n is fixed, but the pair ofavailable materials are set, then FIG. 3 would be used with the knownvalue of n (from the values for the elastic moduli for these materials)to choose a value of m such as to yield a value of f(n, m) as close tothe maximum value of 1.5 as possible. This maximum occurs when m=nHaving thereby chosen the ratio of thicknesses of the laminae (a ainspection of Equation 10 dictates that h, which is the sum of a and abe as small as feasible in view of the application.

When there are no limiting criteria, that is, no predetermined valuesfor n, m or h, the design may easily be effected by trial and error bytentatively choosing a pair of materials providing a large Am. Next thegraphs of FIG. 3 would be employed to seek the best available value form. If the best value of mi available from this initial choice ofmaterials does not suit, it may be necessary to repeat the process witha new choice of materials.

However reliable the derivation of mathematical relationships of designfactors may be in the provision of means for selecting optimum pairs ofmaterials for producing desired constructions of the bimaterial strips,the reliability of such design selections can only be retained byunifying the two selected materials without the introduction into thesystem of a bonding layer such as a glue, cement or other adhesive, orthe introduction of mechanical fastening means, such as rivets, becauseby the inclusion of additional components beyond the two-componentsystem considered in the design the sensitivity (curvature per degree oftemperature change) of the thermosensitive element is reducedsubstantially 'below the sensitivity available by the use of these samematerials bonded in the manner described herein.

Thus, in order to avoid this pronounced disadvantage of the prior artconstructions it was decided to completely eliminate the use of aseparate bonding agent by using a suitable thermoplastic material as atleast one of the laminae and bonding this thermoplastic lamina directlyand permanently to the second component of the system by heating thethermoplastic to the extent necessary to melt the surface thereof at theinterface and then allowing the components to cool together.

One mode for the preparation of a plastic-metal thermo-sensitive elementis as follows: a sheet of his phenol A polycarbonate resin (described inUS. Patent No. 2,946,766 in the names of Schnell et al. issued July 26,1963) is placed upon a sheet of clean aluminum foil, the two laminae areurged into close contact with each other, by a biasing force, thebiasing force is removed, the pair of laminae are heated to atemperature above the softening transition temperature (about for thepolycarbonate film) to melt the plastic and then the combination ofmaterials is allowed to cool. Once cooled, a satisfactoryplastic-to-metal bond results.

In one particular construction an aluminum sheet 1 mil in thickness wasemployed in combination with bis-phenol A polycarbonate film 8 mils inthickness. After heating 5 and then cooling as described above, theintegrated laminate was easily cut to produce strips of the desiredsize.

In the case of various other plastic-metal combinations that may beeffected, the surface of the metal lamina may first be roughened beforecleaning the metallic surface thoroughly to insure adequate bonding withthe melted plastic.

Strips embodying one or more plastic laminae will not withstand exposureto temperature above the glass transition of the particular plastic orplastic laminae employed (about 140 C. for bis-phenol A polycarbonate)and will which values for E and a have not been included herein are; forexample, polyethylene terephthalate (Mylar), polyphenylene oxidepolymers in general (of which polyzylylene oxide has been noted above)as described in US. patent application Ser. No. 212,128, Patent3,306,875 filed in the name of Allan S. Hay on July 24, 1962--andassigned to the assignee of this invention, polymethylmethacrylate,copolymers of vinyl chloride and vinyl acetate (Vinylite resins) andethyl cellulose. If desired, the-metal lamina may be replaced by athermosetting plastic possessing sufiicient strength and flexibility inthin layers.

The parametric values for a series of suitable plasticmetal bimaterialcombinations are catalogued below with the subscript 1 referring to themetal lamina and the subscript 2 referring to the plastic lamina:

*The value given for the thickness ratio m is the one corresponding tothe maximum value 01](n, m) as discussed above, wherein m=n- In the fourcases shown, the metal strips are, in order, about )6, As, A4, and itsas thick as the plastic stri s.

is the total thickness a1+a2.

D TFrom Eq. (10) above, the maximum curvature (It) is 1.5Aa-AT/h, thuskh/Aci=l.5Au, It

not exert as much force as a bi-metallic strip of equal size, but suchbimaterial strips are adequate for the uses disclosed herein, can bemade at a fraction of the cost of bimetallic strips and can be providedin a wider range of sizes.

Among the metals suitably employed in combination with a plastic laminain the construction of a thermosensitive laminate are the following, forexample:

TABLE I [Values are given for 27 C.]

Metal A B o D E F G E BN 2* 410 340 210 135 100 69 200 pliant 13 0] 5.04.5 1.6 5.6 8.0 23 16 *Units: llBillion Newtons/m."]=10 [dyne/cm.-]=1.6X10 p.s.i.

ey. A-Tungsten. B-Molybdenum. CInvar (63.8% iron, 36% nickel, 0.2%carbon). D:Fernico (53.0% iron, 29% nickel, 17% cobalt). E-Titanium.F-Aluminum (Al is included as a reference and also because of itsconvenient ilorm as aluminum foil).

G-Stee Some polymers appropriate for use in bimaterial strips are, forexample:

4 High density polyethylene. 5 Po1ytrifiuorochloroethylene (Kel-F). 6Bis-phenol A polycarbonate (Lexan resin manufactured by General ElectricCompany).

7 Polystyrene.

The notation of fourteen exemplary metals and plastics, leading to atotal of 49 metal/plastic combinations (and 42 plastic/plasticcombinations) is not intended to be an exhaustive list. Additionalplastic materials for If all of the strips have the same value of m thenthe results for kh/ AT will be very different from those shown in thepreceding table. This is because f(n,m) 1.5 unless m =n In FIG. 4 anapplication is shown of the use of the bimaterial strip of thisinvention in a temperature-indicating device. As shown a pair ofadjustable-mounted bimaterial strips 21 and 22 are mounted as shown toeither side of and at the rear of pivotally-mounted mirror 23 with thedistal end of each strip 21 and 22 in contact with the rear of mirror23. The strips 21 and 22 are identical in composition, physicalproportions and cali bration and are mounted so that any change intemperature from the zero temperature at which the strips 21 and 22 arestraight will cause these strips 21, 22 to deflect in the samerotational sense. Mirror 23 is pivotallymounted on bearings 24, 26 to berotated about a vertical axis in response to deflection by one or theother of strips 21 or 22 upon a change in temperature. Bulb 27 withheated filament 28 directs rays of light to mirror 23. Because of itsconcave configuration mirror 23 focuses an image of the filament 28 as aline of light upon translucent screen 29 at a location depending uponits position about its axis is of rotation, which position depends inturn upon the ambient temperature. A proper temperature index scale 31is imprinted upon the surface of screen 29 such that the image offilament 28 is superimposed on scale 31. Screen 29 or scale 31 isproperly located relative to the zero position of mirror 23 and strips21 and 22 and, after calibration, this device provides an accurateindication of ambient temperature. Since only one of strips 21 and 22are actually causing rotation of mirror 23 at any instant, a singlebimaterial strip properly linked to mirror 23 will sufiice, if desired.

The device in FIG. 5 is a device wherein a change in capacitance occursin response to a change in temperature. This change in capacitance issensed by capacitance measuring circuit 41, which in turn can be used tocontrol a servomotor (not shown) or other such equipment. With thearrangement shown the capacitance between electrode 42 and bimaterialstrip 43 will increase with an increase in temperature. If desired, twobimaterial strips similar to strip 43 could be employed with the plasticlayers facing each other and the second bimaterial strip taking theplace of electrode 42. By providing contacts to each of the metalelectrode layers of these strips with a wire conductor leading from eachmetal electrode to capacitance measuring circuit 41 about twice as largea change in capacitance per degree change in temperature is produced asin the device illustrated in FIG. 5.

It has therefore been shown that by the practice of the inventiondisclosed herein the full benefits of sophisticated theoreticalconsiderations in the design of birnaterial thermo-sensitive laminatesmay be secured in the final commercial embodiment, a development notrecognized heretofore in the art. As a result a bimaterial strip of lowcost both in material content and process of manufuture may be producedwhich functions to render large deflection with constantreproducibility.

Various modifications are contemplated and may obviously be resorted toby those skilled in the art without departing from the spirit and scopeof this invention, as hereinafter defined by the appended claims, asonly preferred embodiments thereof have been disclosed.

What I claim as new and desired to secure by Letters Patent of theUnited States is:

1. A bimateral thermosensitive element consisting solely of two unifiedstrips, the first strip being a thermoplastic material selected from theclass consisting of bis-phenol A polycarbonate and polyphenylene oxideand the second strip being a metal having a thickness /5 or less thanthe thickness of said thermoplastic material, said strips beingintegrally bonded without the use of a bonding agent to provide a commoninterface to obviate relative displacement during deflection of saidelement with changes in ambient temperature.

2. A bimaterial thermosensitive element as in claim 1, in which thedilference in linear thermal expansivities of the two strips has amagnitude of at least 5 p.p.m./C.

3. A bimaterial thermosensitive element as in claim 1, in which thevalue of the ratio of the thickness of the second strip to the thicknessof the first strip is approximately equal to the reciprocal of thesquare root of the ratio of the elastic modules of the second strip tothe elastic modulus of the first strip.

4. A thermosensitive element consisting of a layer of a thermoplasticmaterial selected from the class consisting of bis-phenol Apolycarbonate and polyphenylene oxide integrally bonded to a layer of ametal having a thickness /s or less than the thickness of saidthermoplastic material and the difference in linear thermalexpansivities of the two layers has a magnitude of at least 5 p.p.m./C.

5. A thermosensitive construction comprising in combination in aplurality of bimaterial elements consisting essentially of an integratedpair of strips, one of said strips being a thermoplastic materialselected from the class consisting of bis-phenol A polycarbonate andpolyphenylene oxide and the other being a metal having a thickness /5 orless than the thickness of said thermoplastic material, one strip havinga major surface area thereof in integrally bonded relationship with amajor surface area of said other strip, to obviate relative displacementduring the deflection of said element with changes in ambienttemperature.

6. A thermosensitive element consisting solely of a layer of aluminummetal united firmly with a layer of bis-phenol A polycarbonate toobviate relative displacement along the interface therebetween duringdeflection of said element with changes in the ambient temperature, saidmetal having a thickness of /s or less the thickness of saidpolycarbonate.

References Cited UNITED STATES PATENTS 2,355,949 8/1944 Boutwell 156-32,561,217 7/1951 Muir 24010 2,573,686 11/1951 Blinn et al. 73363.52,709,147 5/1955 Ziegler 1563 X 2,724,672 11/ 1955 Rubin 156306 X2,999,845 9/ 1961 Goldberg 26047 2,950,266 8/ 1960 Goldblum 260433,141,863 7/1964 Holrn 161l83 X 3,205,122 9/1965 Crawford et al. 1S6306X 3,179,553 4/1965 Franklin 161183 3,291,935 12/1966 Murphy et -al73--363.5

HAROLD ANSHER, Primary Examiner.

U.S. Cl. X.R.

1. A BIMATERAL THERMOSENSITIVE ELEMENT CONSISTING SOLELY OF TWO UNIFIEDSTRIPS, THE FIRST STRIP BEING A THERMOPLASTIC MATERIAL SELECTED FROM THECLASS CONSISTING OF BIS-PHENOL A POLYCARBONATE AND POLYPHENYLENE OXIDEAND THE SECOND STRIP BEING A METAL HAVING A THICKNESS 1/5 OR LESS THANTHE THICKNESS OF SAID THERMOPLASTIC MATERIAL, SAID STRIPS BEINGINTEGRALLY BONDED WITHOUT THE USE OF A BONDING AGENT TO PROVIDE A COMMONINTERFACE TO OBVIATE RELATIVE DISPLACEMENT DURING DEFLECTION OF SAIDELEMENT WITH CHANGES IN AMBIENT TEMPERATURE.